This is a query more than anything.
As I read the documentation both Script:UpdateWorld() and Script:UpdatePhysics() are both initiated by a single call to World::Update(). In fact the physics is a child of world and is always(?) invoked.
So it appears that the separation is more a conceptual one rather than a functional one.
The only reason I can think of is by putting all physics in UpdatePhysics then if (I'm not sure) that is invoked first you would know that all physics updates have been applied before you do any other world level checks/logic.
So is that it or am I missing something important?
I'm feeling rather dense today.